Do You Need an Amp for These?

You can answer this from two numbers on the spec sheet, before you spend anything on either piece. For most headphones and most people asking, the answer is no. A few genuinely need one. Here is how to read them.

The two numbers

Impedance, in ohms, tells you what kind of hungry a headphone is. High impedance, in practice the 300 ohm designs, wants voltage, which is what a battery-powered source is short of. Low impedance wants current. Neither is better, and impedance says nothing about how a headphone sounds.

Sensitivity tells you how hungry. It is decibels of loudness for a fixed input, and some makers quote it per milliwatt, some per volt. Per volt is the useful one, because your phone is a voltage source. To convert from per milliwatt, add about 18 for a 16 ohm headphone, 15 for 32 ohms, 13 for 50, 10 for 100, and 5 for 300. The result is how loud it plays from one volt, and one volt is the number to hold onto.

If the headphone is	Add this to the per milliwatt figure
16 ohms	about 18
32 ohms	about 15
50 ohms	about 13
100 ohms	about 10
300 ohms	about 5

What one volt buys

Apple's USB-C dongle puts out about a volt, and most phones and laptops land near it. Comfortable listening averages 75 to 85 decibels and a dynamic recording peaks 15 or 20 above that. So a headphone that reaches 110 from a volt has room to spare on a phone. One that reaches 100 will play, with the slider at the end of its travel and nothing in reserve. One that reaches 95 runs out of volume before you run out of wanting more.

Two sources change the answer. A MacBook Pro from 2021 on, a MacBook Air from 2022 on, or a recent desktop Mac detects a load over 150 ohms and switches to three volts, nearly ten decibels more. A dongle like the FiiO KA11 gives about two and a half volts, eight decibels the Apple one lacks. A desktop amplifier, even a small one, gives ten volts and up into 300 ohms, twenty decibels over a phone.

Which headphones need one

The figures are the manufacturers' own, converted to loudness at one volt.

Yes. Buy the amp with the headphone. The HIFIMAN Susvara, 83 decibels per milliwatt at 60 ohms, reaches about 95 from a volt; the HE6 Remastered is within a couple of decibels of it. Both play from a phone at a level you might call polite until there is real power behind them. The Susvara Unveiled is a few decibels easier and still belongs here. The Dan Clark Audio Stealth and Expanse get here the other way: at 23 ohms they look fine per volt, but at 86 or 87 per milliwatt they want hundreds of milliwatts for peaks, and a phone is short on current too. The E3 and Audeze LCD-5s are close behind.

The Sennheiser HD 600 is rated 97 decibels from a volt on Sennheiser's own sheet. The HD 650, HD 660S2 and HD 800 S are 102 to 104, the ZMF Auteur Classic and Atrium about 101, and all of them are 300 ohms. Yes from a phone, where they play at a moderate level with the slider at the top; no from a recent Mac or any desktop amp, where they are fully driven. And we would not read much into the six decibels Sennheiser puts between the HD 600 and HD 650. The HD 6XX, which Sennheiser has sold directly for about $220 since Drop closed in March, is an HD 650 in different paint, and all of this applies to it.

It depends, and the way to find out is to turn it up. The Sundara, HE400se, Edition XS and Arya Organic, the ZMF Caldera, the FiiO FT5 and FT7. All reach 107 and up from a volt, which is enough, but all are low impedance with sensitivity in the low to mid 90s per milliwatt, so the phone is finding current it was not built to supply. Play the loudest, densest record you own at your real volume. If the slider is past three quarters, or the loud passages feel smaller than the quiet ones, an amp will change something. At half, keep your money.

No. Everything else, which is most headphones. Every Grado, at 38 ohms. Every Meze; the 109 Pro is the most sensitive full-size headphone here. Every Focal, the Utopia included, which plenty of people pair with a serious amplifier anyway, and that is a fair thing to want rather than a requirement. The Audeze LCD-X, the FiiO FT1, the ZMF Bokeh, the HD 560S and HD 620S. Every wireless headphone; the amplifier is inside. Every in-ear monitor, for the opposite reason: so sensitive that the problem with a source is hiss, not volume. The electrostatics, the Audeze CRBN2 and Dan Clark Corina, need an energizer, not an amp, and make no sound from a headphone jack.

Headphone	From one volt	Verdict
HIFIMAN Susvara, 60 ohms	about 95 dB	Yes. Buy the amp with the headphone
HIFIMAN HE6 Remastered	Within a couple of decibels of the Susvara	Yes
Dan Clark Audio Stealth and Expanse, 23 ohms	86 or 87 dB per milliwatt, so hundreds of milliwatts for peaks	Yes, and a phone is short on current too
Sennheiser HD 600, 300 ohms	97 dB	Yes from a phone. No from a recent Mac or any desktop amp
Sennheiser HD 650, HD 660S2, HD 800 S, 300 ohms	102 to 104 dB	Yes from a phone. No from a recent Mac or any desktop amp
ZMF Auteur Classic and Atrium, 300 ohms	about 101 dB	Yes from a phone. No from a recent Mac or any desktop amp
Sundara, HE400se, Edition XS, Arya Organic, Caldera, FT5, FT7	107 and up	It depends. Low impedance with sensitivity in the low to mid 90s per milliwatt, so turn it up and find out
Every Grado, at 38 ohms		No
Every Meze, every Focal, the LCD-X, the FT1, the Bokeh, the HD 560S and HD 620S		No
Every wireless headphone		No. The amplifier is inside
Every in-ear monitor		No. So sensitive that the problem with a source is hiss, not volume

What an amp does when it is not about volume

Physics first. A source's output impedance should be a small fraction of the headphone's, an eighth is the usual rule, or the tonal balance of any headphone whose impedance changes with frequency, which is most dynamics and every multi-driver in-ear, shifts with it. Apple's dongle is under an ohm and passes. An output-transformerless tube amp like the Schiit Valhalla 3 is built for 300 ohm headphones and is the wrong tool for a 16 ohm planar or a FiiO FH9, on paper, before anyone listens.

Then listening. People describe a properly driven HD 650 as having more grip in the bass and more ease on peaks, and we think that is roughly right. An amp does not turn a headphone you do not like into one you do. "Strictly requires" and "would benefit from" are different sentences. And a bad amp is worse than the jack on a recent phone.

If you do need one

A Schiit Magni Unity drives everything on the yes list except the Susvara and HE6, and gets those two loud enough to tell you whether to go further. Those two want substantially more voltage and current than a Magni Unity has, and choosing for them is a specification question rather than a verdict on how anything sounds. The amplifier guide does that arithmetic.

How to find out for yourself

Plug it into whatever you own and play the record you know best at your real volume. Volume left over and nothing held back: that is your answer. At the end of the slider: bring it to the listening room at Free River Studios in Livingston, where the same passage on a phone and then on an amp, in one sitting, settles it. The room is free and runs by appointment.